Guanylin and uroguanylin are peptides that are expressed at mucosal surfaces and bind to the transmembrane receptor guanylate cyclase C (GC-C). Ligand binding to GC-C, also expressed on mucosal epithelial cells, elevates intracellular cGMP and elicits transmembrane ion movement via the cystic fibrosis transmembrane conductance regulator and the Na+/H+ Exchanger 3. This signaling pathway has direct clinical relevance in that acute illness caused by enterotoxigenic strains of E. coli is often mediated by the heat stable toxin STa, a super-agonist of GC-C. Using gene deleted mice as our model system, our preliminary data indicate that signaling through GC-C has an important role in regulating mucosal immune homeostasis. We further show that cGMP levels in the epithelial of the intestine are required for regulated, well-controlled NF-?B and that deletion of GC-C, and a subsequent drop in cGMP levels, results in a highly sensitized mucosal surface. Proinflammatory gene expression is elevated in mice with diminished mucosal cGMP levels and this is further exacerbated during lipopolysaccharide challenge. Further, we demonstrate that signaling through GC-C is important in host defense during infection by gastrointestinal pathogens such as Citrobacter rodentium. The overarching hypothesis of this application is that GC-C, via ligand-induced cGMP generation, is an essential regulator of epithelial NF-?B activity and that loss of this aspect of GC-C function deregulate mucosal innate immune homeostasis. We will address this in the following aims. Specific Aim 1 will determine the role of GC- C signaling in NF-?B signal transduction and gene expression. Specific Aim 2 will define the role of GC-C in bacterial pathogen infection. The studies in this proposal will define a novel and important link between GC-C regulated cGMP production and maintenance of mucosal immune homeostasis.